How long has this been happening? Ever since you downloaded the browser?
If yes:
This might just be a bad install, you can try reinstalling.
If no:
It might have been caused by a recent update, check your browser's state at opera://update. If it is up to date, try recovering with the first recovery option selected at opera://update.
@v4mp1r0 If you can access using a VPN, then it (usually) means that either Twitter is being blocked in your region or your IP is being blocked by Twitter.
I don't know how about the rest of the extensions, but Tampermonkey for some reason is considered a malicious extension. so I am using the beta version of Tampermonkey and it does not turn off.
There is already a "Force Dark Pages" setting that makes every website to force dark mode. you can use it but when you use it, you cannot use QR codes because it makes it dark so it breaks. Good luck 🙂